In an active matrix liquid crystal display (LCD), an image is produced by controlling the light transmittance of a two-dimensional array of discrete image elements (pixels), via the conversion of digital image data, consisting of a data value for each pixel of the image, into analogue voltages with values dependent on that data, and direction of those voltages to each pixel electrode in the array via an active matrix of source data lines, gate lines and thin film transistor (TFT) switching elements. This type of arrangement is illustrated in FIG. 1 (from Ernst Lueder, Liquid Crystal Displays, Wiley and Sons Ltd, 2001, and more fully described in that publication). In such an arrangement each pixel may be defined as each separate electrode area in which is addressable via the active matrix with a single data voltage in each display refresh period. Commonly in such displays, a pixel is provided at each intersection of the source and gate lines, and is electrically connected to these lines by a TFT, such that it may be addressed by the application of a gate voltage pulse via the gate line to the gate terminal of the TFT, and during the period of the gate pulse being present, a data voltage via the source line to the source or drain terminal of the TFT. The pixel electrode is connected to the remaining source or drain terminal of the TFT, and may therefore be charged to the data voltage supplied by the source line during the gate selection pulse, but may remain unaffected by previous or subsequent voltages supplied by the source line when the gate voltage pulse is not present. The resolution of such a display is the number of independently controllable image elements, and is determined by the number of intersections between the source and gate lines. In a colour display, there are typically three colour sub-pixels per composite white pixel (one each of red, green and blue type), and each of these needs to be independently controllable in order for each pixel to be able to display any colour or luminance within the display's gamut, so for a colour display a resolution given as m rows by n columns, the number of source and gate intersections is m×n×3. For the purposes of this disclosure we will consider a pixel to be any electrode region corresponding to a source-gate intersection.
In large area (>10″ diagonal) LCDs for applications such as TVs and signage, especially those employing a vertically aligned nematic (VAN) liquid crystal (LC) mode, it is common for each pixel of the display, to be provided with two separate pixel electrode regions, each region being driven by a separate TFT, although both TFT's being connected to the same source and gate lines, and each region being associated with a separate storage capacitor (CS) line. With this arrangement, although the data voltage (VD) supplied to both pixel regions in each frame period will be the same, the signal applied to each capacitor line is separately controllable to allow a different modification to the voltage on each pixel electrode region to be applied subsequently to the application of the common data voltage and removal of the gate selection pulse. This pixel arrangement is known as capacitively coupled split-pixel driving or multi-pixel driving (MPD) and is illustrated in FIG. 2 (a). The equivalent electronic circuit with voltage references is given in FIG. 2 (b).
The advantage of such an arrangement is that is allows each pixel to produce two regions of differing transmittance despite the application of only a single data voltage. This may allow an improvement in the wide viewing angle performance of the panel, and the design and usage of such a pixel arrangement for such purposes is disclosed in U.S. Pat. No. 7,079,214 (Sharp). A timing diagram showing how the voltages supplied to a pixel arrangement of FIG. 2 may be controlled to provide differing transmittances from the two pixel regions, via capacitive coupling of the voltage change applied to the storage capacitor lines after removal of the gate pulse voltage, onto the pixel electrode, is given in FIG. 3.
One potential limitation of the driving methods described in U.S. Pat. No. 7,079,214 is that, although a different transmittance may be produced by the two pixel regions, their relative transmittance has a fixed relationship. As the two storage capacitor lines are common to all the pixels in a row, any modifications made to the pixel electrode voltages after the application of the data voltage to both regions is made to all pixels in the row. Therefore if region A of one pixel of the row is made brighter than region B, then this is true for all pixels in the row (driven with the same voltage polarity, the relative brightness while still fixed being inverted for pixels driven with the opposite polarity), and the capability of providing different transmittance from the two regions does not provide an increase in display resolution. Neither the degree of difference in brightness between the two regions, nor which region is the brighter can be varied from pixel to pixel, so it is not possible to represent image data at a level of detail finer than the pixel array of the panel.
U.S. Pat. No. 4,973,135 (Canon) discloses a means of displaying image data with resolution greater than the number of source-gate intersections of the active matrix of the display, by providing multiple counter electrodes per pixel electrode region. However, in order for a genuine resolution increase to be provided by this method, it must be used in conjunction with a fast-switching, bistable liquid crystal mode, such as a ferroelectric LC.
WO200124153_A1 (ITL) discloses a similar means of achieving multiple regions of independently controllable brightness from each TFT addressed region of an active matrix panel, again utilising multiple counter electrodes per pixel. However, this method is applied to current driven emissive display types, rather than the voltage controlled transmissive LCD, and diode-like behaviour of each pixel allows the counter electrode voltage to select or de-select corresponding regions of the pixel. Also, in order to achieve a genuine resolution increase, each counter electrode controlled region of the pixel must be driven in a time-sequential pattern, so the resolution increase comes at the expense of overall brightness.
WO2011118423 discloses a means of driving an MPD type LCD of the type of U.S. Pat. No. 7,079,214 in such a manner that the different pixel regions may be prevented from transmitting any light regardless of the data voltage supplied to the pixel electrodes, by controlling the voltage on the storage capacitor lines. Again however, the corresponding pixel regions of all pixels of the row must also be prevented from transmitting, so any resolution increase must again come in the form of time sequential transmission from the different pixel regions, and therefore at the expense of brightness.
US20100097366 A1 (Sharp) discloses a means of driving a display with pixels of the type of U.S. Pat. No. 7,079,214 whereby an ac waveform with multiple voltage levels per video frame input period are supplied, in order to allow the wide view improvement provided by the MPD to be achieved in conjunction with a column polarity inversion dc balancing drive pattern, or block polarity inversion drive pattern. Again however, due to the storage capacitor lines being common to all pixels in a row, no means of changing which of the pixel regions is the brighter for different pixels in the row, and therefore allowing the display of higher resolution image data is described.